Pokémon’s success over the last 30 years has seen it grow from a much-loved Game Boy classic, to a world-dominating brand with TV shows, spin-off games, toys and even a theme park. The trading card game is a huge part of that, with more than 85bn cards printed over its lifetime, and the appetite for them shows no signs of slowing down. Packs get snapped up after release so quickly it’s now almost impossible to find them in shops, and a booming resale market is driving the hobby into murky waters.

One card recently sold for an eye-watering £12m, and an increasing number of people have gotten into the collecting business on the hunt for huge profits, leading to a rise in thefts targeting card shops. More recently though, some small businesses have reported that their sales have been hit by thieves targeting envelopes within Royal Mail, leading to thousands in lost revenue.

Ben Thyer, owner of Bath TCG – a card specialist store in the Bath area, is one of those affected. “We always get the odd parcel going missing, but because it was sporadic, we put that down to unfortunate incidents,” Thyer tells the Guardian. “Then [in late July], we had a message [from a customer] with pictures of a cut-open envelope, and the cards were missing. Then there was a second, and then a third, and then there were seven – all of them from the same batch shipped on the same day. We asked for images from each person, and every envelope had been Sellotaped … we don’t use Sellotape because we have brand new envelopes, and we use our own labels over the seals. It’s clear someone had done it deliberately.”

The images, seen by the Guardian, show that the mailed envelopes were tampered with in the exact same way. Thyer considers it clear that this was a targeted theft rather than a coincidence. Fortunately, those particular cards weren’t of high value, but he has since had to close down online card sales, considering the risk of theft and damaged reputation to his business to be too high. By stopping those sales he has now lost out on £4,000-6,000 of potential revenue, a huge amount for a small business.

Bath TCG is not the only card sales business to be affected, and the issue is not just affecting Pokémon cards. Daniel Stack, director of Mana Rock Games – a specialist in selling Magic: The Gathering cards – has also had issues with Royal Mail items going missing. His business is also based in Bath and was affected over similar dates to Bath TCG’s.

“I realised it was theft when two customers in different parts of the country sent me pictures of their packages already open with the contents missing, within minutes of each other,” Stack said. “We’d previously had one sent to France turn up the same way a couple of weeks before, but I put it down to the buyer taking a bit of a chance that I’d have no way to hold them to account for pretending there had been an issue. In total we’ve had about £120-worth stolen so far – we had previously sent over 1,200 orders without any issues.”

Stack also halted online sales for a week, costing him £500 in revenue. Other sellers in the area have mentioned similar problems to him. Unfortunately, as most card businesses in the UK use CardMarket to sell online, it’s the buyer who picks the delivery method, and most often they use Royal Mail services. Cards are often sent in distinctive hard-backed envelopes to protect the contents, often with shop branding on stickers or seals.

Royal Mail is investigating the issue and liaising with the affected businesses. A spokesperson told the Guardian: “The safety and security of mail is a top priority and we take any reports of theft very seriously. We investigate concerns thoroughly and take action where necessary, including working with law enforcement.” Investigations are still ongoing; in the meantime, businesses must choose between risking theft and damaged reputation for their stores, or halting online sales and losing revenue.

With a new, highly sought-after 30th anniversary Pokémon card set due to release this month, retailers are already anticipating an increase in scalper and reselling activity, leading to even more potential for thieves to target those selling cards online. We asked the Pokémon Company for comment on how they’re dealing with increased demand and the rise in thefts, but they have yet to respond.